The invention relates to a tunable filter arrangement. The invention further relates to a transmitter, a receiver, a mobile telephone device, and a cordless data transmission system with a tunable filter arrangement, as well as to a tunable bulk acoustic wave resonator.
The stormy developments in the field of mobile telephony and the continuous miniaturization of cordless telephone devices lead to higher requirements being imposed on the individual components. Thus a high selectivity in the high frequency part is necessary for protecting the receiver from the rising number of potentially interfering signals from other systems. This is achieved, for example, by means of bandpass filters which transmit only a limited frequency band and which suppress all frequencies above and below this band.
At the present moment, filters with ceramic electromagnetic resonators are among the means used for this purpose. A miniaturization of these filters, however, is limited by the electromagnetic wavelength. So-called surface acoustic wave (SAW) filters built up from surface acoustic wave resonators can be given a considerably smaller construction. This is because the acoustic wavelength is smaller than the electromagnetic wavelength by 4 to 5 orders of magnitude. A surface acoustic wave resonator comprises a piezoelectric layer on which finger-shaped electrodes are provided. A signal applied to the input electrodes excites the piezoelectric material into mechanical vibrations, which propagate in the form of acoustic waves on the upper side of the layer and are converted back into an electric signal again by the output electrodes.
An alternative is formed by bulk acoustic wave (BAW) filters comprising bulk acoustic wave resonators. Bulk acoustic wave filters have advantages as regards their size, power, and IC compatibility. Bulk acoustic wave resonators are built up from three components in principle. The first component generates the acoustic wave and comprises a piezoelectric layer. Two electrodes arranged above and below the piezoelectric layer represent the second component. The third component has the task of acoustically insulating the substrate from the vibrations generated by the piezoelectric layer.
It is an interesting aspect that the properties of a resonator or filter can be varied. This may be done, for example, through coupling of a resonator or filter with a varicap diode. It is a disadvantage of the combination of active and passive components that the active components may be contaminated by the materials of the passive components during the manufacture of the resonator or filter.
An alternative possibility is disclosed in U.S. Pat. No. 5,446,306. This describes a semiconductor bulk acoustic wave resonator and a semiconductor bulk acoustic wave filter which comprises a semiconducting substrate, a first and second electrode, and arranged therebetween a piezoelectric layer of AlN or ZnO. The resonance frequency of the resonator is changed in that a DC voltage is applied to the electrodes.
The invention has for its object to provide a tunable filter arrangement which can be manufactured in a simple and inexpensive manner.
This object is achieved by means of a tunable filter arrangement which comprises a substrate and provided thereon an arrangement of at least two resonators coupled to one another, of which resonators there is at least one which contains a piezoelectric component of a ferroelectric material and to which a DC voltage source is connected.
When a DC voltage is applied to the first and the second electrode of a resonator, the electrical properties of the resonator, the resonance frequency, and the anti-resonance frequency will change. If the resonator is in a filter arrangement, the above will have an influence on the total filter characteristic. Ferroelectric materials have a much stronger dependence on an applied voltage as regards their dielectric constant xcex5 than have, for example, AlN and ZnO. This means that the electrical properties of a resonator having a piezoelectric component made of a ferroelectric material are influenced much more strongly by the application of a DC voltage, so that the tuning range of the entire filter arrangement is substantially greater.
It is preferred in particular that the ferroelectric material is chosen from the group comprising PbTi1xe2x88x92xZrxO3 (0xe2x89xa6xxe2x89xa61) with and without dopents of La, Nb or MN and with and without excess lead, LiNbO3, LiTaO3, PbNb2O6, Pb1xe2x88x92xCaxTiO3 (0xe2x89xa6xxe2x89xa61), [Pb(Mg⅓Nb⅔)O3]xxe2x80x94[PbTiO3]1xe2x88x92x(0xe2x89xa6xxe2x89xa61), BaTiO3, Ba1xe2x88x92xSrxTiO3 (0xe2x89xa6xxe2x89xa61) with and without dopants, Ba1xe2x88x92xSrxTiO3 (0xe2x89xa6xxe2x89xa61) with and without addition of VOx (1xe2x89xa6xxe2x89xa62.5) and/or SiO2, Ba1xe2x88x92xSrxTiO3 (0xe2x89xa6xxe2x89xa61) with MgO dopants, [Ba1xe2x88x92xSrxTiO3]xe2x80x94Pb1xe2x88x92yCayTiO3 (0xe2x89xa6xxe2x89xa61, 0xe2x89xa6yxe2x89xa61), Ba1xe2x88x92xSrxZryTi1xe2x88x92yO3 (0xe2x89xa6xxe2x89xa61, 0xe2x89xa6yxe2x89xa61) with and without dopants, Ba1xe2x88x92xPbxTiO3 (0xe2x89xa6xxe2x89xa61) with and without excess lead, Ba1xe2x88x92xCaxTiO3 (0xe2x89xa6xxe2x89xa61), (Pb,Ba,Sr)(Mg⅓Nb⅔)xTiy(Zn⅓Nb⅔)1xe2x88x92xxe2x88x92yO3 (0xe2x89xa6xxe2x89xa61, 0xe2x89xa6yxe2x89xa61), K1xe2x88x92xNaxNbO3 (0xe2x89xa6xxe2x89xa61), (Cd,Na)NbO3, (Bi,Na)TiO3, (Bi,Na,Pb,Ba)TiO3, Bi7Ti4NbO21, (Ba1xe2x88x92xSrx)2NaNb5O15 (0xe2x89xa6xxe2x89xa61), (Ba1xe2x88x92xSrx)2KNb5O15 (0xe2x89xa6xxe2x89xa61), (Ba1xe2x88x92x+y/8Srx+y/8)2Na1xe2x88x92yNb5O15 (0xe2x89xa6xxe2x89xa61, 0xe2x89xa6yxe2x89xa61) with and without excess Na+, (Ba1xe2x88x92x+y/8Srx+y/8)2K1xe2x88x92yNb5O15 (0xe2x89xa6xxe2x89xa61, 0xe2x89xa6yxe2x89xa61) with and without excess K+, (Ba1xe2x88x92xSrx)2K1xe2x88x923ySEyNb5O15 (0xe2x89xa6xxe2x89xa61, 0xe2x89xa6yxe2x89xa61, SE=ion from the group of rare earths), Sr2Ba4Ti2Nb8O30,
a) Pb(Mgxc2xdWxc2xd)O3,
b) Pb(Fexc2xdNbxc2xd)O3,
c) Pb(Fe⅔W⅓)O3,
d) Pb(Ni⅓Nb⅔)O3,
e) Pb(Zn⅓Nb⅔)O3,
f) Pb(Scxc2xdTaxc2xd)O3 and combinations of the compounds a) to f) with PbTiO3 and/or Pb(Mg⅓Nb⅔)O3 with and without excess lead.
It is preferred that the resonators are chosen from the group of bulk acoustic wave resonators and surface acoustic wave resonators.
Filter arrangements with bulk acoustic wave resonators or surface acoustic wave resonators can be manufactured with a high quality factor Q and a high coupling factor k.
It is particularly preferred that the resonators are constructed in the thin film technology.
The construction of the resonators in thin film technology on a substrate renders it possible to obtain such a filter arrangement with small dimensions.
It is particularly highly preferred that the bulk acoustic wave resonators each comprise a resonator unit of a lower and upper electrode and a piezoelectric layer arranged therebetween and a reflection element which is positioned between the substrate and the resonator unit.
Such a bulk acoustic wave resonator can be manufactured without cumbersome lithographic processes because the resonance frequency of the resonator is defined by the layer thickness of the piezoelectric layer. In addition, such a bulk acoustic wave resonator is clearly more robust than other types of bulk acoustic wave resonators such as single-crystal resonators, resonators with membranes, or resonators with air gaps.
The invention further relates to a transmitter, a receiver, a mobile telephone device, and a wireless data transmission system provided with a tunable filter arrangement which comprises a substrate and provided thereon an arrangement of at least two resonators which are coupled to one another, of which resonators there is at least one which has a piezoelectric component made of a ferroelectric material and to which a DC voltage source is connected.
The invention further relates also to a tunable bulk acoustic wave resonator which comprises a substrate and provided thereon a resonator unit having a lower and upper electrode as well as a piezoelectric layer arranged therebetween, and a reflection element which is positioned between the substrate and the resonator unit, to which resonator a DC voltage source is connected.
The strong dependence of the dielectric constant xcex5 on an applied voltage means that the electrical properties of the bulk acoustic wave resonator can be tuned over a wide range.